The present disclosure relates generally to switching-mode power supply (SMPS), and more particularly to SMPSs using primary side control.
Most of electronics appliances require power supplies, which provide output power sources with specific output ratings. For example, an AC-to-DC power supply could convert a mains alternating-current (AC) power into a direct-current (DC) output power source whose output voltage and current are well controlled. Among the power supplies in the art, SMPS is known to have compact size and excellent conversion efficiency, and is popularly adopted in the art.
To prevent users from high-voltage stress or shock from a mains AC power source, a power supply could be designed to have two sides galvanically isolated from each other, meaning there is no direct current flowing between the two sides. One side directly connected to the mains AC power source is generally referred to as a primary side and the other as a secondary side. The voltages on the primary side all reference to an input ground of the mains AC power source while those on the secondary side all reference to a floating ground on the secondary side.
A SMPS with galvanical isolation could provide a pulse-width-modulation (PWM) signal to control a power switch, so as to constrain the electric power transferred from the primary side to the secondary side, and to make an output power source on the secondary side fit all required output ratings. For instance, one output rating requires the output voltage of the output power source to be regulated to be about 5V within a certain tolerance range all the time. The tolerance range is from a bottom limit to a top limit, or from 4.8V to 5.2V for example.
Two kinds of voltage control methodology for SMPS are known in the art; one is called primary side control (PSC), the other secondary side control (SSC). Operating under PSC, Circuitry on the primary side senses a reflective voltage of an inductive device to generate a PWM signal for a power switch while the reflective voltage represents an output voltage on the secondary side. PSC has the circuitry on the primary side indirectly sensing the output voltage. Nevertheless, SSC uses circuitry on the secondary side to directly sense the output voltage and to send information, via a photo coupler for example, to the primary side.
To reduce switching loss of a power switch, the switching frequency of a PWM signal controlling the power switch is intentionally reduced to be very low when a load of a SMPS is light or absent. In other words, during the condition of light load or no load, the ON time TON of the power switch when it is turned ON to conduct current is brief, and the OFF time TOFF of the power switch when it is turned OFF lasts for along time. It is always a challenge to keep the output voltage from falling and to stay within the tolerance range when the light load or no load of a SMPS suddenly turns into a heavy load, however. More particularly, a SMPS with PSC, when its load is light or absent, cannot acknowledge the variation of its output voltage most of time because the reflective voltage only exists briefly when the inductive device of the SMPS is de-energizing. It is an important task to make a SMPS respond fast to the output voltage transient, or the output voltage might drop down below the bottom limit of the tolerance range, one of the output ratings of the SMPS.